Vitamin B12 (cobalamin) deficiency in man leads to two broad classes of clinical phenomena: 1) megaloblastic anemia and related changes in rapidly proliferating tissues and, 2) a variety of neurological disorders. The biochemical basis of these chemical manifestations is not at all understood and indeed the mechanism of the cobalamin-dependent enzymatic reactions has not yet been elucidated. Our proposal outlines experiments that are designed to provide information about the reactivity of the carbon-cobalt bond of the corrinoid coenzymes. What interaction with the enzyme causes the reversible homolysis of this organometalic bond? How does the enzyme prevent the very reactive radical intermediates from spurious reactions? We have selected the adenosylcobalamin-dependent ribonucleotide reductase system as the target enzyme system. The proposed studies are directed towards: a. The characterization of the active site of the ribonucleotide reductase from Corynebacterium nephridii. b. The synthesis and evaluation of possible transition state analogs that will inhibit ribonucleotide reduction and thus cell proliferation. c. 13C-NMR studies of thioredoxin, modified with [13C]methyl iodide and [13C]formaldehyde. d. The purification and characterization of thioredoxin reductase from C. nephridii.